The present invention relates to an STTD encoding method and a diversity transmitter.
STTD (Space time block coding based transmit antenna diversity) is signal processing optionally adopted by a base station in a CDMA communication system when sending information pertinent to a Downlink Dedicated Physical Channel (Downlink DPCH) to a mobile station in open-loop mode.
According to the third-generation cellular telephone global standard system IMT2000 (International mobile telecommunication 2000) technical specification, a transmitter at the base station is required to have a function of carrying out STTD encoding.
The STTD encoding function is explained on page 15 (5.3 Downlink physical channels) of the technical specification (3G TS 25.211 V3.1.1 (1999-12)) specified by the 3GPP (3rd Generation Partnership Project).
As described above, STTD encoding is performed when a base station performs diversity transmission to a mobile station in open-loop mode.
The base station carries out transmission, for example using two transmission antennas. At this time, from one antenna data is transmitted with predetermined encoding carried out. From the other antenna, data is transmitted without encoding.
As an example, suppose a case where symbol S1 is sent immediately followed by S2.
From one antenna, data S1 and S2 are directly transmitted.
From the other antenna, data is sent with the polarity of the imaginary part of a transmission symbol reversed and positions of adjacent transmission symbols switched round. That is, S3 (=xe2x88x92S2*) and S4 (=S1*) are sent. Here, asterisk xe2x80x9c*xe2x80x9d means a relationship between conjugate complex numbers.
The mobile station (receiving side) receives a signal transmitted by one antenna (or a plurality of antennas). The voltage level of a reception signal fluctuates under the influence of fading.
A signal sent from one antenna of the base station (that is, signal whose symbol to be sent is sent as it is) and a transmission signal sent from the other antenna (that is, signal sent with the polarity of the imaginary part of the transmission symbol reversed and positions of adjacent transmission symbols switched round) have different fading modes.
Furthermore, by adding predetermined decoding processing to the reception signal, it is possible to distinguish whether the reception signal is sent from one antenna of the base station or from the other antenna.
Therefore, the mobile station can select a reception signal with greater signal amplitude or combine both reception signals to compensate the reduced amplitude of the reception signals caused by fading.
That is, the receiving side can substantially perform diversity reception even if it has only one antenna.
STTD encoding should not be carried out for a pilot symbol.
This is because a pilot signal is necessary on the receiving side to acquire reception synchronization and encoding the pilot signal itself would make this initial synchronization acquisition difficult.
Therefore, it is necessary to perform STTD encoding on all symbols other than the pilot symbol.
A system configuration to perform STTD encoding is described in FIG. 8 on page 15 of the technical specification (3GPP TS 25.211 V2.4.0 (1999-09).
FIG. 8 shows the configuration (details of FIG. 8 will be described later).
The system shown in the technical specification divides composition of a transmission frame into two stages.
That is, in the first stage, the system multiplexes data on which STTD encoding should be performed and applies STTD encoding to the multiplexed data.
Then, in the second stage, data not to be subjected to STTD encoding (that is a pilot signal) is multiplexed with the data subjected to STTD encoding. This completes composition of one frame.
However, a system carrying out such processing has a frame composition process divided into two stages. Thus, two multiplexers are required for data multiplexing, which will increase the size of the circuit.
Furthermore, frame composition requires strict timing control. Especially using two multiplexers requires accurate timing control, which will constitute a considerable restriction on the system design of the base station.
For example, it is mandatory that two multiplexers and STTD encoder be placed close to each other. This reduces the degree of freedom of the design of a system board and the degree of freedom of the location of each system board.
Here, it is also possible to conceive another configuration using one multiplexer and distinguishing input data to be subjected to STTD encoding from input data not to be subjected to STTD encoding inside the multiplexer.
However, this again requires a configuration for distinguishing input data and performing encoding on only data requiring STTD encoding, which will increase the size of the circuit.
This will also require strict timing control to multiplex data not to be subjected to STTD encoding with data subjected to STTD encoding, which will complicate the circuit. Moreover, strict timing control will constitute a considerable restriction on the system design.
The present invention has been implemented to solve these problems and it is an object of the present invention to reduce the size of the circuit of a system carrying out STTD encoding and avoid any considerable restriction from being imposed on the system design.
The present invention applies reverse STTD encoding to a data part not to be subjected to STTD encoding beforehand.
Then, symbols subjected to reverse STTD encoding are multiplexed with other symbols to be subjected to STTD encoding using a multiplexer to compose a frame.
Then, STTD encoding is carried out on all data included in the composed frame.
As a result, the data subjected to reverse STTD encoding is restored to a state of data not subjected to STTD encoding.
Using such a method eliminates the need to distinguish between data to be encoded and data not to be encoded inside the STTD encoder. Moreover, just one multiplexer will suffice.
Reverse STTD encoding follows the principle, which is totally reverse to that of STTD encoding.
Suppose there are two adjacent QPSK symbols, S1 and S2. Applying reverse STTD encoding results in symbols S1 and S2 being directly output as signals corresponding to one transmission antenna. Moreover, S3 and S4 are generated and output as signals corresponding to the other transmission antenna which have a relationship of S3=S2* and S4=xe2x88x92S1* (here, xe2x80x9c*xe2x80x9d denotes a relationship between conjugate complex numbers).
The present invention contributes to simplification of the configuration of a base station system (transmission system) in a mobile communication.
Furthermore, even if the mobile station (receiving side) has only a single antenna, performing diversity transmission using STTD encoding allows the mobile station to actually carry out diversity reception, which will improve the quality of a reception signal.